The present invention relates to braking systems and more particularly to braking systems incorporating both a dynamic and a mechanical brake. The dynamic brake is preferably a hydrodynamic brake of the kind which includes a control mechanism for adjusting the dynamic braking moment which the dynamic brake applies to a rotating shaft to be braked. The amount of dynamic braking moment which can be obtained by the dynamic brake (as represented by a braking moment/rotary speed diagram) lies inside an operating range which is defined by a limiting characteristic curve for the maximum dynamic braking moment. A control device compares the actual value of a parameter to be controlled (for example, the dynamic braking moment) with an adjustable reference value representative of the desired value of the parameter to be controlled and generates an adjustment value signal which varies as a function of the difference between the actual value and the reference value of the parameter. The adjustment value is applied to the control mechanism of the dynamic brake and controls the operation thereof.
A mechanical brake, which has a control mechanism for adjusting the mechanical braking moment which the mechanical brake applies to the rotating shaft to be braked, is only enabled when the required braking moment exceeds the operating range of the dynamic brake. In this case, the braking moment of the mechanical brake is controlled to be equal to the amount by which the required braking moment exceeds the operating range of the dynamic brake. A braking device of this kind, wherein the dynamic brake is constructed as a hydrodynamic brake, is described in German Patent Specification No. 2,239,008.
As is known, with increasing rotary speed and under otherwise constant conditions, the braking moment produced by a dynamic brake rises according to a parabolic curve. Thus, the characteristic curve which limits the operating range of the dynamic brake in the lower rotary speed range, is a parabola. Above a certain rotary speed, the characteristic curve runs approximately horizontally or rises slightly or falls slightly, since in this range the maximum dynamic braking moment must be kept less than the theoretically possible braking moment so that mechanical or thermal over-loading of the dynamic brake is avoided. As a rule, the dynamic brake should pass through the operating range along specific characteristic curves. A regulating device is provided for this purpose. By way of example, if a hydrodynamic brake is used, the regulating device adjusts the filling level of the hydrodynamic brake.
For this purpose, the known braking device described in the foregoing German Patent Specification has a filling level regulating valve and a device for measuring the dynamic braking moment actually produced. The movable valve component of the filling level regulating valve is constructed as a force comparator which compares a force (the actual value) which is proportional to the actual dynamic braking moment measured, with a force representing the required braking moment (the reference value). If there is a difference between these values, the valve component of the filling level regulating valve is adjusted in such a way that the actual value corresponds to the reference value.
If the rotary speed of the brake rotor decreases so much during a braking operation that the parabolic part of the limiting characteristic curve is exceeded, and the dynamic braking moment which can be obtained is therefore less than the required amount, the mechanical or friction brake must also be utilized. The same may also be the case when the required braking moment lies above the limiting characteristic curve and the rotary speed of the rotor is high. As long as the dynamic brake can produce the required amount of braking moment on its own, the friction brake remains disabled, to keep the wear of the friction surfaces as low as possible.
In the known braking device, the friction brake is automatically switched in as follows. A force corresponding to the actual dynamic braking moment is compared with a force representing a reference value in a three-pressure control valve. If the reference force is greater than the actual force, the control value actuates the operating mechanism of the mechanical brake, i.e. it opens a valve which supplies pressure medium to an operating cylinder in the friction brake so that the mechanical brake actuates. The mechanical brake is preferably actuated in such a way that the braking moment produced by it is as nearly as possible equal to the difference between the required braking moment and the actual dynamic braking moment. In other words, the sum of the braking moments of the friction and dynamic brakes should always be equal to the required amount of braking moment.
In practice, however, it has been found that with this type of combined braking system the desired ideal method of functioning is frequently delayed, at least for a certain time. Very often the mechanical braking moment is at least temporarily higher or lower than is required to supplement the dynamic braking moment. This disadvantage can perhaps be tolerated when braking a vehicle, since in this case the driver of the vehicle can compensate for any possible error by adjusting the brake pedal. However, if, for example, a stationary plant is involved in which the braking device cannot be constantly supervised by operating personnel, the foregoing disadvantage can result in considerable operating problems. Typical examples of such stationary plant are:
(a) a conveyor system comprising several conveyor belts running one behind the other but driven individually. In this instance, when the system is stopped, all the conveyor belts should cover the same braking distance, irrespective of their load; otherwise, there would be a danger that a conveyor belt which was still running would spill some conveyed goods onto the next conveyor belt, which may already be stationary. PA0 (b) a conveyor belt being used for carrying personnel as well as materials. In this instance, official regulations prohibit the exceeding of a specific rate of retardation. PA0 (c) A conveyor belt is conveying material downhill. In this instance, its conveying speed must be held constant by continuous braking, irrespective of variations in loading. PA0 (a) whenever the rotary speed of the dynamic brake is no longer sufficient to produce the required amount of braking moment, and PA0 (b) whenever the mechanical or thermal loading of the dynamic brake exceeds the tolerated level.
A primary object of the present invention is to provide a combined braking device, comprising a dynamic brake and a mechanical brake, which can adjust to a required operating value, such as a specific amount of braking moment or a specific rotary speed or a specific level of retardation, even during combined dynamic and mechanical braking.